Humidity measurements are important in many industrial operations including food processing, the manufacture of plastic, paper and fine chemicals and in the production of semiconductors. In several biochemical processes where water activity or moisture levels are vital to the metabolic productivity of microorganisms, moisture levels must be determined in a fast, accurate, reliable manner, and on a real-time basis. Over the past few years, different types of humidity sensors have been developed and the recent trend has been to place such sensors in an in-line system for the detection and control of moisture levels.
Generally, moisture sensors incorporate a sensing material with an appropriate transducer. Sensing of moisture is achieved by interaction of the water molecules with the sensing material and the resulting electrical or optical signal is relayed to a detection system via a transducer. For example, Hijikigawa et. al in "A Thin-Film Resistance Humidity Sensor", Sensors and Actuators, 1983, 4, pages 307-315 have developed a humidity sensor consisting of a thin film coating on alumina substrate. The film is a composite made from crosslinked polystyrene-sulfonate covered with a protective film of cellulose ester. The film electrical resistance is sensitive to humidity.
In another example, Y. Sakai et al. "Humidity-Sensitive and Water-Resistive Polymeric Materials", Sensors and Actuators, 1988, 16, pages 359-367 discuss a two-polymer system comprising polytetrafluoroethylene vinylpyridine graft copolymer (PTFE-VP) and crosslinked polyvinylpyridine with 1,4-dibromobutane. The impedance of the two-polymers changes with the relative humidity.
As an example of ceramic or metal oxide based sensors, Y. Sadoka et. al "Electrical Properties of Anodized Aluminium in a Humid Atmosphere", Journal of Materials Science, 1986, 21, pages 1269-1274 have investigated anodized aluminium for the measurement of humidity. On the other hand, S. Mukode et al. in "A Semiconductive Humidity Sensor", Sensors and Actuators, 1989, 16, pages 1-11, discuss the use of semiconducting materials such as stannic and titanic oxides.
Cobalt chloride (CoCl.sub.2) has been extensively used as a specific optical indicator for humidity. Anhydrous cobalt chloride is blue and changes to pink when hydrated. Generally, cobalt chloride based sensors have limitations and different concentrations of cobalt chloride are needed to measure a wide range of humidity levels. Other limitations include poor stability and reproducibility, variable accuracy, a limited temperature range, and a slow response time. Cobalt chloride based sensors may not be appropriate in many industrial situations.
Russell et. al in "Optical Sensor for the Determination of Moisture", Analytica Chimica Acta, 1985, 170, pages 209-216, report embedding cobalt chloride into a gelatin matrix and coating the matrix onto a 600 .mu.m optical fiber to produce a humidity sensing system. The change in relative humidity is related to the absorption of light by CoCl.sub.2 at 680 nm. The light is transmitted to the fiber and is then reflected internally.
Ballantine et. al in "Optical Waveguide Humidity Detector", Analytical Chemistry, 1986, 58(13), pages 2883-2885, also use cobalt chloride, in conjunction with a glass capillary tube. The cobalt chloride is incorporated into polyvinylpyrrolidone (PVP). The sensor is coupled to light emitting diodes and calibrated by monitoring the change in light absorption by cobalt chloride at a given wavelength.
Zhou et. al in "Porous Fiber-Optic Sensor for High-Sensitivity Humidity Measurements", Analytical Chemistry, 1988, 60(20) pages 2317-2320 discuss a humidity sensor using cobalt chloride incorporated into a porous glass fiber with a high surface area. The sensitivity of the sensor is dependent on the concentration of cobalt chloride. At low concentrations of cobalt chloride, the sensor is able to measure at low relative humidity. By increasing the concentration of cobalt chloride, the sensor is able to measure high relative humidity.
In other applications, Harris et. al in "Colorimetric Detection of Humidity and Other substances with Solvatochromic Dyes Dispersed in Porous Polymer Films", NASA Tech Briefs, MFS-26128, discuss thin polymer films containing solvatochromic dyes sensitive in their UV-Visible absorption spectrum to atmospheric water content. Detection of a color change can be achieved visually by comparison with a standard chart or spectrophotmetically by measurment of the wavelength of absorbance.
In other applications, sensors based on fluorescence have been used. Generally fluorescence-based moisture fiber optic sensors have demonstrated poor sensitivity especially at low relative humidity and respond slowly to changes in relative humidity. These limitations are due to the dynamic range of the reagents or dyes used. For example, Posch et. al in "Fiber-Optic Humidity Sensor Based on Fluorescence Quenching", Sensors and Actuators, 1988, 15, pages 77-83, report a fiber optic humidity sensor based on fluorescence quenching. Two different fluorescent dyes are used as humidity indicators: perylenedibutyrate and perylenetricarboxylic acid bis-imidates (PTCABs). A silica gel sheet is used to absorb the dyes and is subsequently cast onto a glass slide which is attached to a bifurcated fiber optic light guide to form a sensing system. Sensor sensitivity is poor at the high range of relative humidity and the response time is slow. Furthermore, gases such as oxygen and ammonia interfere with the desired, water mediated quenching mechanism.
Zhu et al. in "A New Fluorescence Sensor for Quantification of Atmospheric Humidity", Journal of Electro Chemical Society, 1989, 136, pages 657-570, report a fiber optic humidity sensor. The fluorescent dye rhodamine 6G (R6G) is impregnated in a Nafion polymer and is sensitive to changes in relative humidity. The sensor has limited sensitivity at relative humidities below 40%.